1. Field of the Invention
The present invention relates to a composition for a resin which is advantageously used for optical materials, such as plastic lenses, prisms, optical fibers, substrates of information recording materials and filters, particularly as a material of plastic lenses of glasses.
2. Description of the Related Arts
Plastic materials have widely been used as various optical materials, particularly as lenses of their glasses, because of light weight, toughness and easiness of tinting. The properties required for optical materials, particularly for lenses of glasses, are a low specific gravity, optical properties such as a large refractive index and a large Abbe number and physical properties such as high heat resistance and large strength. A large refractive index is important to decrease thickness of a lens. A large Abbe number is important to decrease chromatic aberration of a lens. High heat resistance and large strength are important to facilitate fabrication and also from the standpoint of safety.
As conventional materials having a large refractive index, thermosetting optical materials having a thiourethane structure which are obtained by the reaction of a polythiol compound and a polyisocyanate compound have been proposed (Japanese Patent Publication Heisei 4(1992)-58489 and Japanese Patent Application Laid-Open No. Heisei 5(1993)-148340). Technology to obtain a lens by polymerization of an epoxy resin or an episulfide resin with a multi-functional compound has also been proposed in the specifications of Japanese Patent Application Laid-Open Nos. Heisei 1(1989)-98615 and Heisei 3(1991)-81320 and International Publication No. WO8910575. Of course, optical materials having further large refractive indices are desirable.
A small chromatic aberration is another important property required for an optical material. The larger the Abbe number, the smaller the chromatic aberration. Therefore, a material having a large Abbe number is also desirable. Thus, a material having a large refractive index and a large Abbe number is desired.
However, the Abbe number tends to decrease with an increase in the refractive index. Plastic materials obtained from conventional compounds have the maximum Abbe number of about 50 to 55 when the refractive index is 1.50 to 1.55, about 40 when the refractive index is 1.60 and about 31 when the refractive index is 1.66. When the refractive index is forced to increase to 1.70, the Abbe number decreases to 30 or less and the obtained material cannot practically be used.
Moreover, in the case of conventional materials, particularly materials having a thiourethane structure, the molecular weight of a sulfur-containing compound used as the raw material must be increased to achieve a large refractive index and this decreases crosslinking density. The content of alkyl groups must be increased to achieve a large Abbe number and this decreases the rigidity of molecules of raw material compounds. As the result, drawbacks such as a decrease in heat resistance arises. In other words, the refractive index of conventional optical materials obtained from episulfide compounds, polythiol compounds and isocyanate compounds can be increased only to a limited degree. Moreover, an increase in the refractive index decreases the Abbe number. Therefore, an advantageous balance between a large refractive index and a large Abbe number cannot be achieved by conventional optical materials.
To solve the above problems, the present inventors discovered novel sulfur-containing compounds having an episulfide structure from which optical materials having a small thickness and a decreased chromatic aberration can be obtained (Japanese Patent Application Laid Open Nos. Heisei 9(1997)-110979 and Heisei 9(1997)-71580). The present inventors further discovered resin compositions which give optical materials showing improved oxidation resistance and tint performance from those of optical materials obtained by curing the above sulfur-containing compounds by polymerization (Japanese Patent Application Nos. Heisei 9(1997)-104724 and Heisei 9(1997)-333120). However, optical materials obtained from these compositions do not have a sufficient impact resistance. When an optical material has a small impact resistance, the optical material is required to have a thickness large enough for providing a practical strength. This causes a critical problem for decreasing the thickness and the weight even though optical materials having a decreased thickness can be designed with respect to the high refractivity index.